A differential assembly of the type contemplated for the present invention is generally of the design shown in U.S. Pat. No. 2,859,641 (GLEASMAN). This patent is incorporated herein by reference to the extent necessary to provide specific details of the structure of the differential assembly.
This type of differential includes a rotatable gear housing, a pair of drive axles received in bores formed in the sides of the housing, and a differential gear arrangement mounted within a main body portion of the housing for driving the axles. The gear housing body portion includes a flange formed at one end for mounting a ring gear or other means for providing power input to the differential from the drive shaft of the vehicle in a conventional manner. The gear housing is provided with a cap at its other end which may be formed as an integral part of the housing or may be removably secured to the housing.
The gear arrangement which is referred to as a "crossed-axis compound planetary gear complex" includes a pair of helical worm or side gears coupled to each axle end, together with so called transfer gears associated with each of the side gears and in mesh with each other for transferring and dividing torque between the axle ends. The transfer gears are mounted in pairs within slots, or windows, formed in the main body portion of the gear housing, and each transfer gear of a pair rotates on an axis of rotation that is substantially perpendicular to the axis of rotation of the side gears and gear housing.
The transfer gears are in reality combination gears, i.e., the middle portion of each gear constitutes a worm wheel portion while the outer ends of the gear are formed with integral spur gear portions. The gear arrangement is such that, for any given pair of combination gears, the worm wheel portion of a first combination gear meshes with one side gear while the worm wheel portion of a second combination gear meshes with the other side gear, and the spur gear portions of the respective combination gears mesh with each other.
In one example of this type of differential assembly, a set of three combination gears are arranged with their respective axes of rotation in a first plane at approximately 120.degree. intervals about the periphery of each side gear, each of the three combination gears being paired with a combination gear of a second set of three combination gears similarly arranged with respect to the second side gear in a second plane parallel to the first plane.
Each of the combination gear pairs may be considered as a part of a separate gear train for interconnecting the side gears. Although it would be possible to transmit a limited amount of power between side gears using only a single gear train, one or more additional pairs of combination gears are used to define additional gear trains for increasing power transmitting capability of the differential and to provide additional frictional surfaces for dividing torque between drive axles. However, once a first gear train is used to operatively connect the side gears, relative rotational positions of the two side gears are established and additional gear trains must be assembled in a manner which preserves this established rotational relationship between side gears. Otherwise, the additional trains will not fit properly into mesh.
The relationship between the gear trains which is required to fit each gear train properly into mesh with the two side gears may be referred to as "timing." In known designs, it has been necessary to mount each combination gear in a particular order and at a distinct rotational position with respect to the other combination gears to maintain the required timing relationship between gear trains. Such particular rotational positions cannot be readily ascertained from the mere appearance of the combination gears, and the large number of possible mounting positions which do not meet the required timing requirements renders trial and error assembly procedures impractical. There is also the danger that it would be possible to fit pairs of combination gears into mesh at inexact positions which would prevent an equal sharing of loads by the different trains.
Accordingly, special assembly procedures have been used to ensure that the gear trains are properly assembled. Examples of such procedures are found in U.S. Pat. Nos. 3,849,862, 3,875,824 and 3,902,237 (all to BENJAMIN). These patents disclose special procedures for mounting the combination gears in a particular order and rotational position within the gear housing. Reference marks are placed at the same relative position on each combination gear and the reference marks are used to index the partial rotation of each combination gear by a predetermined amount with respect to the other combination gears to arrive at an exact timing relationship between the gear trains.
These procedures, however, are cumbersome and time consuming. Typically, it is necessary to identify the reference mark on each combination gear and to mount each gear in a particular rotational position and in a prescribed sequence with respect to each of the other combination gears. Different procedures may also be required for each new gear design which is used within the differential, and partial reassembly of the gear trains to replace one or more gears in the assembly poses additional problems which cannot be easily accommodated by the established procedures.
Modifications to the known differential type such as those disclosed in prior U.S. patent application Ser. No. 06/895,870 also affect the timing relationship between gear trains. The prior application discloses a gearing arrangement whereby combination gear pairs are shifted relative to each other along the axial length of the side gears to distribute contact wear over a larger area of the side gears. The shift in relative positions of combination gear pairs along the axial length of the side gears requires modifications to established assembly procedures which may further complicate the already difficult procedures.
A further gear-timing relationship is involved in this same differential gear assembly, and it relates to the common practice of indexing (i.e., rotating a predetermined proportion of a single revolution) a first spur gear portion of each combination gear with respect to the second spur gear portion of the same combination gear. This relative indexing is done to promote a smooth transmission of power between members of each combination gear pair. For example, U.S. Pat. No. 3,735,647 (GLEASMAN) discloses concentric first and second spur gear portions of each combination gear which are indexed relative to each other by predetermined amounts selected to avoid the relative position of one-half circular pitch (i.e., one-half of the circumferential distance from one gear tooth to the next). Such timing relationships associated with the relative indexing of the spur portions of each combination gear require that the two members of each combination gear pair be separately designed. For instance, if the first spur portion of one member of a combination gear pair is relatively indexed by one-quarter pitch in a clockwise direction with respect to the second spur portion of the same combination gear, then the first spur portion of the other member of the combination gear pair must be relatively indexed by one-quarter pitch with respect to its associated second spur portion in a counterclockwise direction. Since two different designs of combination gears are required in such differential assemblies, further complexity and cost is added to the manufacture and assembly of the differentials.